Welcome to the forum
I haven't heard of a surgeon recommending two different brands on the same person. Was this because they the 425cc wasn't made by the other manufacturer? (I know that's a very specific size so my guess is not everyone makes it.)
Cosmetically speaking, you will most likely be happy with the results as long as the surgery goes as planned. But I think the question should be asked is if silicone is really the route you want to go? Ever since being re-approved by the FDA a few years ago they have been all the rage, but they are still real dangers involved if they leak. Often times when the leak, the person is not even aware of it. Do you play sports or other strenuous activities that may make the saline breast implants more prone to leaking?